A unity response control system is a system whose closed loop response has unity gain at frequencies below the closed loop bandwidth of the loop. The closed loop bandwidth is defined as the frequency at which the loop gain first drops to -3 db.
The performance of conventional control loops is limited by (a) a loss of forward path disturbance and noise rejection at frequencies above the closed loop bandwidth of the system, and (b) by a lack of rejection of feedback path disturbances below this frequency. Conventional control loops are also limited in bandwidth as a result of an inability to readily compensate for undesirable dynamics in the control loop. For the purposes of this invention disturbances are considered to be external inputs to a system that may or may not be measurable. Noises are considered to be unmeasurable, internally generated signals.
Prior adaptive control concepts, such as that described in "Nonlinear Automatic Control" by John E. Gibson, MacGraw Hill Book Company (1963) are limited by external, forward path or feedback disturbances that degrade plant identification or model reference calculations. In prior model reference adaptive control approaches the adaptive controller converges to a best fit of the plant dynamics and the disturbance, or alternatively, it converges to a best fit of their inverses. This often results in a desired behavior not being obtained, or the system exhibiting unstable behavior.
Conventional control loop bandwidth extension is limited by a requirement to tailor both a phase and a gain of a control compensation device and the fact that conventional compensation filters do not allow independent adjustment of their phase and gain characteristics. However, an adaptive filter can exhibit approximately independent phase and gain characteristics. Thus, an adaptive filter can be used to compensate a closed loop so as to achieve a bandwidth unachievable by conventional means.
By example, for a control system having a resonance located at or just above a desired closed loop cross-over frequency, a conventional notch filter cannot be used, while Adaptive Noise Cancellation (ANC) is well suited for compensating the system. Such is described, for the open loop case, in an article entitled "Adaptive Noise Cancelling: Principles and Applications" Proceedings of the IEEE, Vol. 63, No. 12, December 1975, by B. Widrow et al. It should be noted that an adaptive notch as described in this article will, in many cases, cause a closed loop control system to be unstable unless compensation techniques are employed as taught in commonly assigned U.S. patent application Ser. No. 07/075,013, filed Jul. 17, 1987, entitled "Adaptive Noise Cancellation in a Closed Loop Control System" by J. M. Alcone.
Also by example, control loop noise floor reduction by conventional filtering techniques is limited by the phase restrictions inherent in closed loop systems. Such- noise is particularly troublesome when the noise spectrum lies within the bandwidth of the loop. Conventional ANC practice for open loop applications requires a measure of the noise to serve as a reference. However, such a measure is usually unavailable when dealing with a noise floor problem in a closed loop system, since the noise is generated within the components of the loop and is not independently measurable.
FIG. 1a shows a conventional control system of the prior art having measurable forward and feedback path disturbances, d and n, respectively. FIG. 1b is a graph illustrating the asymptotic disturbance rejection characteristics of the conventional control loop of FIG. 1a. In FIG. 1b it is assumed that the control system is a TYPE 1, first order plant. For simplicity the disturbance (r/d) or error rejection (e/c) and closed loop bandwidths are shown as being identical. It should be noted that the forward path disturbance (d) is attenuated at frequencies below the error rejection or closed loop bandwidth (BW) while the feedback path disturbance (n) is attenuated at frequencies above the error rejection BW. The open loop (OL) and closed loop (CL) transfer functions (TF) are as shown. Also shown is the loop noise floor which, for the purposes of this invention, is considered to be the quiescent state loop noise which is essentially flat and extends across the entire spectrum. An ideal control loop is considered to be one having a unity gain transfer function between an external input and the loop output.
In the aforementioned journal article entitled "Adaptive Noise Cancelling: Principles and Applications" Proceedings of the IEEE, Vol. 63, No. 12, December 1975, by B. Widrow et al. there is described the concept of adaptive noise cancelling, the concept being developed in the context of an open loop system. The disclosed technique employs a primary input containing a corrupted signal and a reference input containing noise correlated in some unknown way with the primary noise. The reference input is adaptively filtered and subtracted from the primary input to obtain a less corrupted signal.
In a journal article entitled "Transform Domain LMS Algorithm" IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-31, No. 3, June 1983, S. by S. S. Narayan et al. transform domain adaptive filtering is described. Narayan et al. employ comb filters to separate, or orthogonalize, inputs to an adaptive filter weight calculation to improve adaptive filter performance for open loop applications.
In U.S. Pat. No. 3,932,818, Jan. 13, 1976 Masak discloses an adaptive filter for suppressing narrow band noise in wide band systems. In U.S. Pat. No. 3,961,234, Jun. 1, 1976 Chambers et al. teach an adaptive filter circuit and an adaptive "dead band" to make the loop less responsive to noise and a control loop error signal. In U.S. Pat. No. 4,238,746, Dec. 9, 1980 McCool et al. disclose an adaptive line enhancer for spectral line enhancing. In U.S. Pat. No. 4,524,424, Jun. 18, 1985, White teaches a matched pair of filters acting on reference and error inputs for enhancing an adaptive filter's ability to detect a signal within a noisy spectrum. In U.S. Pat. No. 4,589,137, May 13, 1986 Miller discloses an adaptive filter and presents only an open loop application. In U.S. Pat. No. 4,568,426, Apr. 14, 1987, Chabries et al. disclose a noise suppression device embodied as a feedback suppression device wherein a projection operator has an output fed back for adjusting filter weights. In U.S. Pat. No. 4,730,343, Mar. 8, 1988 Kanemasa et al. disclose a method for extracting a reference noise signal from an error signal.
In U.S. Pat. No. 4,473,906, Sep. 25, 1984 Warnaka et al. disclose an active acoustic attenuator that operates with a modified version of the LMS algorithm. Closed loop applications are not disclosed and, in addition, Warnaka et al. disclose a requirement that a disturbance include DC and low frequency components, and that the LMS algorithm be modified to reflect the acoustic delays inherent to the system.
It is an object of this invention to provide Adaptive Noise Cancellation methods and apparatus for lowering a noise floor of an open loop or a closed loop control system, for extending the error rejection bandwidth of a closed loop control system for improving the rejection of feedback path disturbances, and for achieving a desired improved control characteristic in a control system that is substantially insensitive to noise and other disturbances.
A further object of the invention is to provide noise floor suppression that does not require a direct measurement of the disturbance, that requires but one adaptive filter, and that is suitable for use with open or closed loop systems having time delays or other dynamics.
It is a further object of the invention to provide method and apparatus for extending a disturbance or noise rejection bandwidth of a closed loop control system, where disturbances or noises can occur in the forward or feedback paths.
A further object of the invention is to provide an adaptive compensation filter in parallel or series with a control loop to provide a unity response control system that is insensitive to measurable, and in some cases unmeasurable, disturbances and noises, and which furthermore has an improved control performance.